30 Sep 04
(2) PBIA accounts for the variations of the random variables and empirical model in barge impact
design. Coefficients of variations for barge impact forces range from around 10 up to 30 percent depend-
ing upon the load condition being considered. The selection of return periods as defined in Table B-1
needs to be tied to the variation of the uncertainties incorporated in the PBIA. The higher the variations in
the input for a load condition, the higher range of the return period should be selected accordingly.
Appendix D shows a PBIA example and how to select return periods for design.
(3) The PBIA method requires that annual distributions should be determined for mass, impact angle,
and approach velocities as well as the uncertainties in the empirical model defined in paragraph B-2. The
uncertainties in mass, velocity, and impact angle can be related to the variations of impact load and the
likelihood of occurrence of loading conditions by using Monte Carlo simulation software (e.g., @Risk) as
described in Appendix D.
(4) Examples of data and distributions for mass, angle, and velocity from recent designs of USACE
navigation projects structures are shown in Appendix C. Appendix D shows the deterministic calculations
for impact force using the empirical equation as defined above as well as an example for a PBIA for an
upper guide wall structure.
B-4. Parameters for Barge Impact
a. Background. It is frequently difficult to estimate the range or distributions of masses, approach
velocities, and angles used in the PBIA. This range should include angles and velocities caused by a loss
of power and control, as well as any anticipated future changes in navigation traffic at the lock. This range
of data should be compiled into a design matrix and processed with return periods for anticipated events
at the particular structure. Return periods were previously discussed in paragraph B-3. For preliminary or
feasibility design efforts, engineering judgment should be used to formulate reasonable impact angle and
velocity scenarios. For some existing locks, the designer may have information available from previous
model studies or lockmaster logs. Other ways to obtain data for feasibility level designs could be from
using lockmaster's logs or towing industry records from similar existing facilities. This type of data
should be utilized only during conceptual design and should not be incorporated as the only source of data
for the final design. Limited data could result in an unsafe or uneconomical design of the navigation
structure. As work progresses toward the final design, the range of values for impact angles and velocities
should be defined with reasonable certainty. Measurements for these parameters can be made in the field
using time-lapse video photography or in a laboratory scale model. Relative merits of each method are
discussed in c(5) and c(7) below.
b. Site constraints.
(1) Approach walls are provided upstream and downstream of lock chambers. Approach walls
adjacent to the dam are commonly referred to as guard walls and the walls opposite the guard walls are
usually referred to as guide walls. The walls are used by approaching barge traffic as landing or holding
points prior to entering the lock chambers. Barge traffic routinely impacts the walls at ranges of velocities
and angles that are constrained by the geometry of the site. This is shown in Figure B-7. Approach walls
are designed to accommodate a wide variety of operating conditions that range from normal river
conditions to flood events. The levels of loading that the walls resist should be consistent with a proba-
bilistic approach where loading is classified as usual, unusual, or extreme based on a given return period
of the event.
(2) Generally, the upstream approach walls are designed for a higher impact load than the down-
stream walls as explained below. Upstream of the lock, riverflow is distributed from bank to bank. The
cross-sectional area of the lock in the river will partially block bank-to-bank flow. To improve hydraulic